In the purification of molten metals, particularly aluminum, it is frequently desired to remove dissolved gases such as hydrogen or dissolved metals, chiefly magnesium. The removal of dissolved gas is known as "degassing", while the removal of magnesium is known as "demagging". Further details concerning the demagging of aluminum are described in an article by M. C. Mangalick, entitled "Demagging Aluminum" which appeared in Die Casting Engineer, January-February, 1974, the disclosure of which is incorporated by reference.
For demagging aluminum, chlorine gas is usually used since magnesium chloride has a more negative free energy of formation than aluminum chloride, so that the chlorine will react preferentially with the magnesium instead of forming aluminum trichloride. Kinetic factors of various prior art methods do not permit the ultimate formation of magnesium chloride. Thus, aluminum trichloride and free chlorine can be emitted into the atmosphere according to the prior art methods. Both of these compounds are air pollutants.
Earlier practices include capturing of pollutants in an enclosed cover connected to a suction generating water treatment plant. Each pound of magnesium reacts with about 2.95 lbs. of chlorine to form MgCl.sub.2, and "demagging efficiency" is therefore defined as 2.95 divided by the actual amount of chlorine used to remove 1 lb. of magnesium. The efficiency of this method of chlorine removal has been less than 75%, and in the worst cases has been 0 in cases of low magnesium content.
Another method of purification of aluminum is described in Derham et al, U.S. Pat. No. 3,650,730, wherein a flux containing a double salt of chlorine, such as cryolite, is used as a chloridizing agent, in removing the magnesium or other impurity. The apparatus of the Derham patent requires maintenance and continuous monitoring of flux composition and thickness, among other variables.
Another form of apparatus for refining molten aluminum is described by M. J. Bruno et al, in U.S. Pat. No. 3,767,382. According to this apparatus, gas is introduced through a rotating hollow shaft and impeller arrangement which presents the problem of maintaining a leak-proof gas-rotating shaft junction.
It is therefore an object of this invention to provide a new and improved method of introducing gas into molten metal such as aluminum, in a manner which permits greater efficiency in the use of introduced gas, and greater control over the escape of the introduced gas into the atmosphere.